:(a^3-a^2-a+1)/(a^3-3a^2+3a-1)
=（a^2（a-1)-(a-1))/(a-1)^3
=((a-1)(a^2-1))/(a-1)^3
=(a-1)(a+1)/(a-1)^2
=(a+1)/(a-1)